This invention relates to the forming of thin films and, more particularly, to forming thin films by vapor deposition.
The technique of thin film deposition is one of the most important processes in the manufacture of solid state electronic devices. In terms of its essential components, this deposition process involves three basic steps: producing a vapor from a source material, transporting the vapor to a substrate, and condensing the vapor on the substrate to form a thin film layer.
In some of the early evaporation techniques, a resistance heated wire, strip, or boat of W, Ta, or Mo was placed in a vacuum chamber to contain or support the material to be evaporated. After sufficient heating, the evaporant condensed and formed a film on a substrate which was positioned in the vacuum chamber near the heated element. This technique, however, exhibited some significant disadvantages. Any heated component, for example, will react with many types of evaporants, particularly oxides, causing decomposition or contamination of the deposited film. Another problem arises when the evaporant is produced from an incongruent material, i.e., one with constituents which exhibit considerably different volatilities. The vapor from such materials will deposit in a film which is not stoichiometric with the evaporant, because the film will include an excessive concentration of the component having a higher volatility.
Flash evaporation was developed to overcome this problem with incongruent materials. In the flash technique, a small quantity of material is rapidly and completely evaporated to produce a film of the same composition as the evaporant. Co-evaporation, where different substances are evaporated at different rates and brought into contact to react at the substrate, has also been used to alleviate the problem of incongruent evaporation. The latter two methods, however, do not provide a solution to the problem of impurities which are introduced by the presence of a heated element in the vacuum chamber.
Further improvements in the film deposition art have been realized by heating only a selected portion of the evaporant material with an electron beam or by ion bombardment (sputtering). These methods have been found to be preferable for the deposition of undecomposed oxides, such as SiO.sub.2, Al.sub.2 O.sub.3, and ThO.sub.2, and for the evaporation of high melting point metals, such as Pt and Ir.
More recently, thin films have been formed by vacuum deposition through the use of a laser located outside the vacuum chamber. This technique is particularly suitable for producing exceptionally pure coatings, since ultrahigh vacuum conditions can be maintained with a thoroughly baked out vacuum system. High purity levels are possible with this approach because the laser heats only a small volume of material located at the focal point of the coherent light beam and thus does not extensively heat the container for the source material. Since the remainder of the vacuum system is not heated, it is possible with a laser deposition process to introduce a variety of gases into the evaporation chamber and thus achieve effects similar to "reaction sputtering", so that this technique may also be utilized to provide doping for the film which is formed. Furthermore, the rate of film formation is not problematic with laser deposition, since deposition rates exceeding 1000 .ANG./min are readily achievable. A particularly attractive feature of the laser technique is that it allows congruent evaporation of some materials which normally evaporate incongruently. This is accomplished because only a small volume of material located at the laser focal point is elevated to a temperature sufficient to produce a vapor stoichiometric with the evaporant, while the conduction of heat away from the focal point is slow relative to this localized heating and evaporation.
Those laser evaporation techniques which are known in the art, however, have exhibited some limitations. One disadvantage of the laser technique, for example, which has been observed in previous laser evaporation work is that the laser tends to cause "splashing" of the source material. This splashing occurs because the high power level of the laser causes the eruption of the source surface with hot solid particles and liquid droplets. This effect results in a deposited film with poor morphology and low crystalline quality. Consequently, a need has developed in the art for an improved laser deposition technique.